Throughout this description and the claims which follow, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.
The principles of indirect evaporative cooling have been well known for many years. Early references to the principle of pre-cooling air through a combination of heat exchange and evaporation prior to evaporative cooling include SU 979796 by Maisotsenko. These principles have been exploited in a number of practical applications as shown in, say, U.S. Pat. No. 4,977,753 by Maisotsenko, and as further refined in U.S. Pat. No. 6,581,402, US Application 2004/0226698 by Reinders and in PCT/AU2006/000025 by James.
The practical devices as shown in the aforementioned disclosures present a number of difficulties to be ameliorated before those devices can operate to a standard that is satisfactory for a commercially viable product.
Of great practical importance in the provision of coolers is their size and shape which must be such as to fit and blend into the surroundings of, say, a domestic dwelling. While it has been traditional to mount direct evaporative coolers on rooves, the additional size and weight of indirect coolers of the same cooling capacity make this approach impractical. A similar problem presents itself when the working models of indirect evaporative coolers are positioned at ground level around the outside of a dwelling. The cooler may have a plan area which can be too large and use too much of the available space between, say, the wall of a building and a boundary fence.
Marketing of indirect evaporative coolers would be greatly enhanced if a cooler could be fitted into a package to be mounted against the outside wall of a dwelling and had dimensions which satisfied the following criteria:                A depth as small as practicable from the wall to the outside of the cooler.        A width limited by handling considerations.        A height only limited by clearance to the underside of any overhanging roof/eave members.        
An ideal configuration, consistent with technical requirements for its operation and, considering the above, would typically project from a wall, against which it is mounted, a distance of no more than about 600 mm, a width of up to 900 mm and height limited to about 2100 mm.
The construction of a cooler within these parameters requires a radical departure from previously disclosed constructions of those devices. In all previous disclosures, certain dimensions of the heat exchanger core are defined by technical and/or practical restraints with only one dimension being able to be varied to increase the capacity of the heat exchanger.
The Maisotsenko device shown in U.S. Pat. No. 6,581,402 has a heat exchanger width and depth restrained by air flow and resistance considerations where capacity is determined by the height of the exchanger. The configurations of Reinders and James have the depth and height of the heat exchanger determined by technical considerations, while the width determines the capacity of the device. We believe that if the favourable characteristics of each of these configurations could be combined into a device, then a much more practicable indirect cooler could be developed for the marketplace.
However, each of the previously disclosed configurations has other technical and practical difficulties which make a ready combination of their advantages somewhat problematic.
The device disclosed in U.S. Pat. No. 6,581,402 requires horizontal heat exchanger plates with a wettable surface on one side and an impervious surface on the other. Water distribution throughout the wettable surface relies on a wickable media distributing water from a central trough with a combination of wicking along the surface and some gravitational assistance from a slight decline from the horizontal. Cooling to a low temperature requires water flow through the wetted surface to be as low as possible, and preferably just sufficient to replace the loss due to evaporation. Flushing of the wettable surfaces to remove any accumulation of salts left behind by evaporating water is not possible without significant degradation of the thermal performance of the air conditioner.
The Maisotsenko configuration, which progressively transfers a percentage of cooled air from the dry passage to the wet passage to provide evaporative cooling, compromises the temperature of the delivered air relative to other configurations, since the transferred air cannot be subjected to the full temperature difference offered by the heat exchanger.
The configuration offered by Reinders fully exploits the original principles of indirect evaporative cooling, but the heat transfer performance of the heat exchanger is compromised by its layout and construction. Heat transferring from the wet passage side to the dry passage side has to travel through relatively long distances of heat exchanger material, necessitating the use of high conductivity materials such as metals to achieve reasonable performance.
In both of the configurations used by Reinders and James, water distribution to the heat exchanger core is by irrigation of the top surface of the core, allowing water to flow down through the core to be collected in a reservoir below the core. This water flow through the core must be kept to a minimum while the cooler is in operation since any excess water flow over that required for evaporation will compromise the temperatures of the usable conditioned air that can be delivered by the cooler. The water flow requirement for thermal performance desirably includes the capability for it to flush residual salts from the core surfaces. A reasonable, practical compromise utilised by both Reinders and James is to periodically wet the core with an excess of water to flush out residual salts and fill the water retaining capability of the materials used to form the wet channels, followed by relatively long periods of operation without wetting. During this period, evaporation still takes place from water held in the wetted surfaces and full thermal performance of the cooler is achieved. This solution works well provided the wetting period is short, and the period between wettings is long.
An alternative revealed by James is to divide the core into separately wetted segments, each segment with its own, thermally separated, water circuit (pump, reservoir and distributor). This method allows for constant flushing of the core without degradation of thermal performance. While this alternative has been demonstrated to work in practical models, it is difficult to implement in viable production models.
A preferred solution is therefore to periodically wet the wet passages of the core with periods as long as possible between wettings to allow evaporation to take place and maximise the cooling capacity of the heat exchanger. This ideal becomes more difficult as the height of the heat exchanger is increased to take advantage of the preferred configuration of the overall indirect evaporative cooler. Water can only be added to the top of the core at a rate determined by the time it takes to trickle down through the wet passages. The wetting cycle must continue until water has spread through most of the vertical distance of the core, and to then be cut off allowing the excess water to flow all the way through the core and back to the reservoir. The cooling effect of the heat exchanger is compromised all the time that water is flowing through the wet passages, and only reverts to maximum cooling when all water flow has stopped. The taller the core, and hence the longer the wet passages, the longer will be the period required for wetting and the greater the proportion of time during which cooling is compromised. This situation becomes untenable with cores as tall as those required for the preferred geometry described above.